Photochromic substances undergo a spontaneous reversible color change on exposure to electromagnetic radiation. For example, a photochromic substance which is transparent and normally colorless will, upon irradiation, turn dark, the degree of darkening being dependent upon the particular substance employed and the degree of irradiation. The basis of photochromic reaction is that photochromic substances, e.g., silver halides, can undergo a chemical reaction wherein species are produced which absorb light to a considerably greater degree than silver halide salts. However, the very essence of photochromism requires that this reaction be reversible so that the substance can again attain its light transmissive state.
While many substances are known which can exhibit photochromism, silver halides are unique and highly desirable for two reasons. First, the photo-induced reactions which these salts exhibit are theoretically fatigue-free, i.e., non diminution of darkening or degree of recovery occurs even after repeated exposures to activating radiation so long as the crystal is properly protected. Second, the light-absorbing activated species produces a grey color in the photochromic material, indicating that substantially all wavelengths of visible light are absorbed.
It is believed that the only successful silver halide based photochromic material are those in which fine particles of photosensitive silver halide are isolated in the substance so that the reaction products are trapped within a microscopic environment where they are available for recombination. One successful substance is a photochromic glass produced by forming minute silver halide crystals in a molten glass matrix and thereafter allowing the glass to cool in a mold. Another successful silver halide based photochromic substance is disclosed in copending U.S. application Ser. No. 544,078, filed Jan. 27, 1975. This application discloses a stabilized photochromic particle comprising a silver halide crystal protected by a coating which is impervious to halogen migration. Since the particles do not interfere with light transmission and since the coating affords effective protection to the silver halide crystals, these particles may be incorporated directly into optically clear prepolymers which may in turn be cast to form polymeric lenses and other opthalmic quality devices.
Attempts to incorporate photosensitive silver halides directly into prepolymers have met with extremely limited success. The reason for this lack of success is believed to be that the silver halide particles are rendered inoperative by the hostile environment present during polymerization or that the structure of the formed polymers allows dissociation of the reaction products produced on activation of the photosensitive particles. While it is possible to achieve a degree of photochromicity in such structures, they have limited commercial value since they are characterized by rapid fatigue, i.e., permanent darkening caused by the buildup of photolytic products.